That means for any shutter speed / aperture dialed on an D800 the results at ISO 1600 setting in a decent raw converter are better compared to any higher ISO setting because noise is the same but highlights are less clipped. If you need shutter speed like at ISO 6400 set ISO 1600 and dial -2EV for the shutter speed. Now you have an option to use a curve in raw conversion to compress highlights instead of just clipping them in the camera.